The FastTrack Railer works quite well. We used it on the modular layout. It saves a lot of time building up a long train. It can even be used with passenger cars by holding one end of the car and rolling the other end down the railer and onto the track. Then lower the second truck onto the railer and roll it onto the track. It also helps to open one coupler on each passenger car.
The OP's question about a track mounted re-railer is a good one. It shouldn't be too hard to make one out of styrene using the HO version as a guide.